Modern uninterruptible power supplies frequently couple two power sources through a single transformer structure to supply uninterruptible power to a load to be energized. A primary power source, typically supplied by a commercial utility, customarily continuously supplies power to the output. A secondary or reserve power source is activated to supply power to the load only upon failure or degradation of the primary power source.
Power flow through a highly reactive transformer is a function of the phase angle between an input signal and the output signal. To achieve high efficiency, the reserve power source is continuously operated in an idling condition, that is, the inverter switches are timed to operate so that the output signal of the inverter is in phase with the output signal of the power supply. Hence during idling, no power flows from the reserve source to the output load. Upon failure of the primary source, the reserve power source responds immediately to supply power to the output load.
Since uninterruptible power supplies are frequently used to supply power to data processing type circuits, the reserve power source must act quickly to supply power whenever the primary power source experiences either a permanent or temporary failure in order to maintain the integrity of information in the data processing circuit.
During the transition period the output signal changes from being in phase with the inverter signal to a signal which lags the phase of the inverter signal. When this occurs, the output signal extends its period temporarily to achieve the desired phase lag and hence experiences a transient frequency change, sometimes called a frequency hit. Such frequency hits may destroy the integrity of information contained within the data processing circuit being powered.